This invention relates to treatment of hot melt ink images and, more particularly, to a system for treating hot melt ink images so as to enhance the quality of the images and, at the same time, prevent cockling and inhibit curling of the substrate which may occur in the processing of the hot melt ink images.
In the preparation of hot melt ink images, improved quality can be obtained by maintaining the temperature of the ink on a substrate above its melting point for a selected time. For example, as described in U.S. Pat. No. 4,873,134 to Fulton et al. which is incorporated herein by reference, heating of a hot melt ink transparency prepared by ink jet printing to a temperature above the melting point of the hot melt ink followed by rapid quenching produces improved transparency projection characteristics. For optimum image quality, the time during which the ink is maintained above its melting point, and the rate of quenching thereafter, should be uniform throughout the image. Moreover, during this process the transparency substrate, which may be made of a sheet of polyester material such as Mylar, for example, may be heated to a temperature that is above the glass transition temperature of the substrate material.
Similarly, as described in U.S. Pat. No. 4,971,408 to Hoisington et al. the quality of hot melt ink images on porous substrates may be improved by maintaining the substrate at a temperature above its melting point for a selected time.
As described in the Spehrley Jr. et al. U.S. Pat. No. 4,751,528, however, when a substrate material passes between a high-temperature region and a low-temperature region, differential thermal expansion of the substrate tends to produce cockle in the substrate. Because of the rapid and extreme temperature changes to which a substrate may be subjected during processing of the type described in the above-mentioned Fulton et al. and Hoisington et al. patents, there is a strong tendency for the substrate to cockle. Such cockling causes separation of portions of the substrate from the heating and/or cooling surface, causing nonuniform heating and/or cooling of the ink drops on the substrate with an accompanying loss of quality of the image. To prevent such cockle, the substrate may be supported on a curved platen.
The response to heating of substrate materials such as transparency substrate polyesters and paper substrates differs and the cockle effect is caused in those substrates in differing ways. When a web or sheet of paper substrate passes from ambient temperature into a heated zone, it expands so that the width of the web increases but, after the paper has been heated for a period of time (typically 5 to 10 seconds), it loses moisture and shrinks, making the web or sheet narrower. On the other hand, the width of a polyester substrate remains larger after it passes into a heated zone so that the cockling effect resulting from such passage must be counteracted or prevented in a different way. During rapid processing of the type described herein, however, the moisture loss from a paper substrate is not significant so that, in general, the same procedures can be used to prevent cockle in both types of substrates during the processing described herein.
When a polyester substrate is kept at a temperature above its glass transition temperature, the substrate loses its flatness memory and tends to conform to the shape in which it is maintained when heated. Thus, where a curved platen surface is provided to prevent cockle, hot melt ink transparencies which have been heated while on the curved surface are formed with a curl which prevents them from lying flat on a projection surface, causing the projected image to be unsatisfactory. Paper substrates may also be curled by such processing.